Decoppering agent

ABSTRACT

A decoppering agent for incorporating into a propellant to remove copper from the rifling of the internal bore of a gun barrel. The decoppering agent comprises a lead-free pulverized additive mixed together with the other propellant compounds and comprises a mixture of tin and bismuth.

This application is a continuation of PCT/SE03/00028, which designatedthe United States filed Jan. 13, 2003, and which in turn claims priorityfrom Swedish application 0200366-3 filed Feb. 8, 2002, the entiredisclosures of which are incorporated herein by reference.

TECHICAL FEILD

This disclosure relates to an addition to every kind of propellanteffective for removing copper deposits from the inside surfaces of gunbarrels. More particularly, a composite addition that has a pulverizeddecoppering agent mixed into the propellant.

BACKGROUND

Most guns have a barrel with a rifled internal bore that imparts astabilizing spin on an expelled projectile. The internal bore may becoated with a hard facing material, such as chromium, to minimizeerosive wear, and thus increasing the number of projectiles that may befired from the gun.

The typical large caliber projectile has a diameter slightly less thanthe diameter of the internal bore. One or more obturator, or rotating,bands gird the circumference of the projectile. At the bands, thediameter of the projectile is slightly larger than the internal diameterof the gun barrel. When the projectile is expelled, the rotating band isengraved by the rifling, contacting the rifling throughout the length ofthe tube imparting the projectile with a stabilizing spin. Projectilesfor rifles and pistols normally do not have a rotating band, but theyvery often have a jacket made out of tombac, which is engraved by therifling.

The gun barrel is manufactured from a material such as steel andsometimes coated with a hard material such as chromium facing. The gunbarrel is harder than the rotating band of jacket, which is typicallycopper or copper alloy. As a result, a portion of the copper from therotating band or the jacket is deposited on the rifling inside the gunbarrel. This copper deposition, referred to as “copper fouling”, canaffect the ballistics of the projectile and major fouling can preventthe projectile from being inserted and seated, positioned in the barrelprior to firing, properly.

Copper fouling is currently a major problem for large artillery weapons,such as 155-millimeter howitzers, and is also noted in small and mediumcaliber cannons, such as 20-millimeter canons. It has become a biggerproblem in rifles today because of the use of lead-free primers. Before,the primer contained a small amount of lead, which worked as adecoppering agent. The current solution to copper fouling is including adecoppering agent into the propellant. The decoppering agent removes thecopper without damaging the gun barrel or the rifling.

A common decoppering agent is a sheet of lead foil deposited between thepropellant and the projectile. On ignition of the propellant charge, thelead is vaporized and diffuses into the copper. The resultant alloy isbrittle and easily shattered. The combination of the heat generated bythe burning propellant and the mechanical movement of the propellantgases separates the brittle lead/copper alloy from the surface of thebarrel. The fractured debris is swept from the muzzle of the gun withthe propellant gases.

A second theory as to why lead foil is effective as a decoppering agentis that the heat generated by the burning propellant melts the leadfoil. Liquid lead contacts the copper deposition and dissolves thecopper, and the copper bearing the lead solution is then expelled as aliquid from the muzzle with the propellant gases.

While metallic lead and lead compounds are effective decoppering agents,the materials are toxic to humans working around the weapons. There is aneed for lead free decoppering agent.

Among the lead free decoppering agents that have been proposed arebismuth, bismuth subcarbonate (BiO)₂CO₃, tin, and tin alloys. Butbismuth compounds are very brittle and even metallic bismuth cannot berolled into a thin foil like lead. Alloys of bismuth metal with othermetals can be rolled into a foil, but the alloys are very expensive andless effective as a decoppering agent. There are also solutions whereone uses specially made pellets of bismuth in a nitrocellulose binder,which are then added to the propellant charge.

There remains, therefore, a need for a method to effectively introduce alead free decoppering agent into the propellant. To include thedecoppering agent in the propellant composition makes it easier andcheaper than having to add a specially made decoppering additive to thecharge.

DESCRIPTION

There is provided a decoppering agent that is incorporated into thepropellant to remove copper from the rifling of the internal bore of agun barrel. The decoppering agent consists essentially of a lead-freepulverized additive mixed together with the other propellant compounds.We have found that a mixture of tin and bismuth is a very gooddecoppering agent. Particularly, the ratio between tin and bismuth isbetween 10-62% Sn and 90-38% Bi, and especially in the ratio 42% SN/58%Bi, the melting point for this combination is 138 degrees C. Thismelting point is much lower than each of the components. The decopperingagent can contain all kinds of suitable tin and bismuth compounds. Somesuitable compounds are selected from the group consisting of metallicbismuth, bismuth alloys, bismuth compounds, metallic tin, tin alloys andtin compounds. The bismuth and tin either vaporizes or liquifies whenthe propellant is ignited and either embrittles or dissolves the copperdeposits facilitating removal.

Accordingly, this disclosure makes possible an essentially lead freedecoppering agent that is included in the propellant. In a particularembodiment, the decoppering agent is a part of the propellant and isdistributed homogenously throughout the propellant.

It is an advantage that the decoppering agent is distributed to thebarrel and trough the barrel with the propellant gases. Including thedecoppering agent in the propellant also means that the powder gasesalways contain the same amount of decoppering agent.

The biggest advantage is that the decoppering agent always is there,thus making the loading easier and cheaper. No need exists for extracontainers with especially made substances for decoppering.

1. A decoppering agent for a propellant, which comprises a lead freepulverized additive of a mixture of tin and bismuth compounds foreffectively removing copper deposits from a gun barrel, wherein thebismuth compounds are selected from the group consisting of metallicbismuth, bismuth alloys and bismuth compounds and the tin compounds areselected from the group consisting of metallic tin, tin alloys, and tincompounds; wherein the ratio between the tin and the bismuth is between10-62% Sn and 90-38% Bi.
 2. The decoppering agent of claim 1, whereinthe mixture contains 42% Sn and 58% Bi.
 3. A propellant compositioncomprising the decoppering agent of claim
 1. 4. A propellant compositioncomprising the decoppering agent of claim
 2. 5. The propellantcomposition of claim 4, wherein the decoppering agent is distributedhomogenously in the propellant.
 6. The propellant composition of claim3, wherein the decoppering agent is distributed homogenously in thepropellant.